Population-Only One Variable First, population growth is only one of several variables that affect the quality of life, however defined. Per capita income, the state of technology, the degree of concentration of human settlements, and the social and cultural diversity of the population are others. There is little doubt that, at least in the developed countries, sheer numbers are not nearly as important in causing pollution as are the high levels of consumption and the by-products of a highly developed and diversified technology. As pointed out in the section dealing with resources, the rise in energy consumption, for example, is due far more to increases in per capita income than to the growth of population, and many adverse effects can be attributed to technical factors that are not inevitable concomitants of energy production and consumption. Thus it would be a gross oversimplification to blame numbers of people alone for the set of problems confronting modern society. Moreover, it is impossible to isolate the effects of a single variable—like population—and picture life in a world adjusted for a different value of that variable unless we allow for inescapably associated changes in other variables. We may shed tears for the adversities or insults that confront us, especially as we compare them with our private visions of what might have been or could be, but we can never know what tears we might have shed had different combinations of factors given us a different world. Preferences and Costs For example, in the United States or western Europe the relatively low price of owning and operating private automobiles, closely associated with the rise of a mass market due to a large population and high incomes, has been a prime factor in making faraway places accessible, but the same automobiles in urban areas produce noxious gases that now befoul the air beyond the air's capacity to dilute or transport them. What is the net effect on the quality of life? Where is the trade-off between newly won mobility and clean air? Would the benefits of overcoming the adverse by-product, through new technology or through the modification of economic incentives, or both, be sufficiently attractive to stimulate manufacturers and users to pay the costs of developing and using products that do not befoul the air? The point is that our reactions tend to be lopsided. In a way, the interest rate by which we discount the future also operates in looking back, but unevenly. The ugliness, dangers, and adversities of the past (called "the good old times") are heavily discounted in comparison with those of the present. Evaluating "trends" becomes a matter of impressions, difficult if not impossible to define in some objective fashion. Thus we are driven to look for more reliable indicators, such as conditions of the environment, natural and man-made, that we can measure, albeit with difficulty.